1. Field of Invention
The present invention relates to a cable-strut roof system, and more particularly to a double-layer cable-strut roof system which comprises a plurality of tension members and compression members arranged in a new manner, and is adapted for exhibition venue, stadium, theater, airport terminal, railway station and other large-span space structure buildings.
2. Description of Related Arts
In recent decades, various types of large-span roof systems are widely used, such as reticulated shell structure constructed of rigid structural members. Reticulated shell structures, however, exhibit high ratio of rise to span, in order to obtain necessary stiffness and good work performance. The structure is heavyweight and more expensive to build with increasing in span.
Lightweight roof structures have gradually been applied with the development of new materials and new technology, such as application of prestressed flexible structures like cable network structures, tensioned membrane structures, and so on. The prestressed system has no stiffness and uncertain shape prior to prestressing. Here flexible system means each internal node thereof receives only flexible tension members such as cables or membranes without rigid compression members. As regards the way forces are transmitted through the system, the internal system is in continuous tension. This structure has the advantages of large-span and beautiful shape, while the internal system must rely on an external supporting system. Only when boundary nodes of the internal system are anchored to an external boundary and a lower supporting system, with their strong support and by prestressing flexible elements, the internal system could be a structure undertaking external loading. The boundary and lower supporting system can only be designed firmly for equilibrating internal tension forces, leading to high cost and a complicated prestressed structure. Another disadvantage of flexible structures involves too large structural deformation under loading.
A self-stressed structure, called a tensegrity structure, has been presented to optimize internal forces distribution, which is a system in a self-stress state and in a stable self-equilibrated state comprising a continuous set of tension members and a discontinuous or continuous set of compression members. Here the self-stress state means that tension members and compression members are connected together with predefined topological relations. During the assembling process, the interaction between members, and the interaction between members and nodes lead to the tension of the tension members and the compression of the compression members. The internal forces of the system do not result from external effect and do not rely on an external supporting system, so that the internal forces are self-stresses. This also indicates that the tensegrity system is an independent system and is essentially different from prestressed system. The stability and self-equilibrium indicate the initial mechanical state of the system, before any loading, even gravitational. The self-equilibrium of the system is in a self-stress state. The stability means that the system is capable of re-establishing its equilibrium position after a perturbation. The stability of the system is closely related to rational topological relations between the two sets of tension members and compression members of the system. Tensegrity structure is also essentially different from traditional structures (such as grid structure, reticulated shell structure, etc.) in members' arrangement and the way forces are distributed within it. It is a system in continuous tension and discontinuous or continuous compression. This mechanical mechanism is a very rational form pursued by engineers in engineering field. But, so far, with the exception of some tensegrity sculpture having the characteristics of art, tensegrity structure hasn't been used in buildings of large-span roof system in the field of construction.
A circular cable truss dome is illustrated in U.S. Pat. No. 4,736,553 to Geiger who has been inspired by the tensegrity principle. This cable truss dome is constructed of a plurality of upper tensioned members, diagonal tensioned members and vertical rigid struts in compression. The upper tensioned members and diagonal tensioned members are radially oriented and attached to an inner tension ring or to the vertical rigid struts, or to an outer compression ring. Several tensioned hoops are affixed to the lower end of the compression members. A flexible membrane is placed on top of the vertical rigid struts to form a roof for the delineated area. This structure is different from cable network structure and prestressed flexible membrane structure as it is constructed of flexible elements such as cables with stiff elements such as compression struts. Combination of stiff elements and flexible elements increase in the stiffness of the structure and overcome a disadvantage of a flexible structure resulting in large deformation under loading. The cable dome structure comprising a plurality of discontinuous compression members is also different from traditional structures such as reticulated shell structure in which compression necessitates the continuity of forces transmission, which efficiently use the tensile strength of cable, tremendously reducing the overall steel consumption and being lightweight. However, this structure does not use triangulated construction, so the structure lacks a degree of lateral stability at the top radial chord of the dome. Furthermore, due to the radial arrangement of the vertical strut, this structure is only appropriate for use in circular plane.
U.S. Pat. Nos. 5,259,158, 5,355,641 and 5,440,840 to Levy utilize a triangulated arrangement of tension members and compression members to construct a roof structure, which are based on the cable dome designed by Geiger. As a result the structure is more appropriate for an oval roof structure. The triangulated roof structure designed by Levy also includes a central truss positioned along the major axis of the oval. Furthermore, the structure can also be designed as triangulated cable dome with annular roof or retractable roof.
Compared with the Geiger system, the Levy system has higher stiffness and structural stability. Both the Geiger system and the Levy system are adapted for spanning large areas for supporting a roof such as arena or stadium for Olympic game. The two systems improved the traditional way that forces are transmitted, which are applicable to span large areas with attractive design. For example, the average steel weight of the Georgia Dome roof designed according to the Levy patent is about 30 kg/m2. The forces transmitted through the two systems are similar, both from the inside such as the innermost tension hoop (or center truss), the vertical struts and cables (including upper cables, tension hoops and diagonal cables) to the outside such as outer upper cables and diagonal cables and finally to the outer compression ring. The outer compression ring receives tension forces resulting from the inner cables of the inner system affixed to it in all directions. The system is built by assembling all components and anchoring the outermost upper cables and diagonal cables to the outer compression ring. Generally, compared with the inner components, the compression ring made of reinforced concrete or prestressed concrete has a huge size. Moreover the compression ring has been a part of the whole building, it is very difficult to identify cable dome structure as an independent structure. As the Geiger system and the Levy system rely on a robust supporting system around and down below, they are still in the scope of prestressed structures and will inevitably have disadvantages of prestressed structure. Furthermore, such domes are costly to build due to node fabrication, construction and installation.
Because of the drawbacks highlighted above of the rigid reticulated shell structure, prestressed flexible structure and cable dome structure, it is necessary to develop a new type of large-span lightweight space structure, which can be simple to construct and have considerable economic benefits, also have innovative features with unique visual effects.